IBM fabs graphene FETs

Portland, Ore. -- Graphene field-effect transistors (FETs) using a single layer of carbon atoms atop a silicon wafer have been successfully fabricated at IBM's T.J. Watson Research Center (Yorktown Heights, N.Y.). Although the technique is a decade away from widespread commercialization, IBM is currently working on radio frequency (RF) applications of the technology for discrete devices planned by the Defense Advanced Research Projects Agency (Darpa).

"We wanted to compare our work on nanotube transistors with graphene FETs," said Phaedon Avouris, an IBM Fellow and the manager of Nanoscale Science at the Research Center. "Their performance is not quite as good as carbon nanotubes, but graphene's electron mobility is at least an order of magnitude [10X] greater than silicon."

Electronically, graphene is a zero-bandgap semi-metal, because its valence and conduction bands overlap. But IBM was able to open up a small bandgap between them by fabricating the transistor's channel from a nanoribbon of graphene just 20-nanometers wide. For its RF devices for Darpa, IBM plans to further shrink its graphene nanoribbon transistor channels to as narrow as two nanometers.

"Unlike silicon, which has a bandgap that you bridge to have electrical transport, graphene does not have a gap, so you can't ordinarily turn it on and off like a transistor," said Avouris. "But we have discovered how to open up a gap by confining the electrons in a very narrow ribbon of material, in a manner similar to a nanotube, producing a quantum wire."

Using conventional e-beam lithography, IBM has successfully fabricated graphene FETs with very narrow nanoribbon channels. So far, the bandgaps opened were relatively small, compared with the excellent properties of nanotubes. This IBM attributes to the imperfections in its method of cutting the nanoribbons. However, by supercooling the device the researchers were able to prove the concept. Next, they plan to further narrow the nanoribbons to achieve room temperature operation.

"Nanotubes and graphene are essentially the same thing, but nanotubes have nice well-defined edges, whereas it is very difficult to cut graphene into well defined ribbons," said Avouris. "But by using unconventional cutting techniques, we hope to narrow the nanoribbons to about two nanometers, which we think will result in a room-temperature graphene FET."

The first applications of the nanoribbon FETs will be for RF devices in Darpa's Carbon Electronics for RF Applications (CERA) program. The high electron mobility of graphene makes it an excellent candidate for analog ultra-high-frequency oscillators and switches.

"We think analog graphene devices will have wide applicability in communications, radar and other areas requiring very-high-frequency operation," said Avouris.

IBM used the mechanical exfoliation method to place graphene atop a silicon wafer for its current device; in the future, they plan to also pursue growing graphene on silicon-carbide wafers. By heating a silicon-carbide wafer in a high vacuum to evaporate the silicon atoms from the top layer, it is possible to leave behind a monolayer of pure carbon in graphene's crystalline lattice.